The user interface must be stabilized at some point, so that documentation writers and translators can work on a fixed target that doesn't obsolete screenshots or documentation.

After the user interface freeze (UIF), the following things must not change any more without approval of the release team and notification (usually via email, with a link to the mail in your exception bug) to the documentation and translation teams:

the user interface of individual applications which are installed by default,

the appearance of the desktop,

the distribution specific artwork,

all user-visible strings in the desktop and applications which are installed by default.